Method of making an encapsulated circuit



y 1966 v. F DAHLGREN Re. 26,017

METHOD OF MAKING AN UNCAPSULATED CIRCUIT Original Filed Dec. 27. 1960 INVENTOR Victor F. Duhlgren A TTORNE' Y United States Patent Office Re.26,017 Reissued May 3, 1966 26,017 METHOD OF MAKING AN ENCAPSULATEDCIRCUIT Victor F. Dahlgren, Woodland Hills, Califi, by SandersAssociates, Inc., Nashua, N.H., a corporation of Delaware, assignor tosaid Sanders Associates, Inc. Original No. 3,186,887, dated June 1,1965, Ser. No. 78,634, Dec. 27, 1960. Application for reissue Oct. 23,1965, Ser. No. 505,143

1 Claim. (Cl. 156-3) Matter enclosed in heavy brackets appears in theoriginal patent but forms no part of this reissue specification; matterprinted in italics indicates the additions made by reissue.

This invention relates to an encapsulated article and method of making,and more particularly, it relates to printed circuits covered with aninsulating material. This application is a continuation in part of myco-pending applications, Serial No. 21,273, filed April 11, 1960, andSerial No. 598,170, filed July 16, 1956, now abandoned, and is a reissueof U.S. Patent 3,186,887, issued June I, 1965.

In the prior art, printed circuits were typically formed by bonding thincopper sheets with a suitable adhesive to an insulated base formed froma thermosetting resin. Selected portions of the copper were etched away,leaving a conductive pattern in the configuration of the desiredcircuit. However, thermosetting resins are characteristicallyhygroscopic, and under high humidity conditions, insulation resistancebetween the conductors was reduced. This material has been replaced inmany uses with trifluorochloroethylene, termed herein Kel-F as atrademark and manufactured by the Minnesota Mining and ManufacturingCompany. This is superior to thermosetting resin material in that it hasa low permeability to humidity as well as an enhanced insulationresistance. Another suitable material, tetratluorochloroethylene, termedherein Teflon as a trademark and manufactured by the Du Pont Company,and other types of thermoplastic material may be used in the practice ofthis present invention. In my co-pending application, Serial No. 21,273,there is disclosed a method of bonding thin copper sheets to a base ofthermosetting material or more broadly, a plastic resin having a fiowcharacteristic at a given temperature. This is done by providing acoating of black cupric oxide to the copper sheet and laminating theplastic and copper together by means of suflicient heat and pressure toform thereby an imprint of the cupric oxide in the plastic. When thelaminated copper plastic has been cooled and the copper removed, thereis exposed a plurality of randomly exposed follicles in the plasticprovided by the imprint. In that co-pending application a surface of asecond plastic resin having a greater tendency to flow at a temperaturesubstantially less than the given temperature was placed in contact withthe imprinted surface. The plastic resins were pressed together at atemperature substantially less than the given temperature to force thesecond plastic resin into the follicles to provide a bond therebetween.

In accordance with the present invention, there is provided a method ofbonding a thermoplastic resin of a given flow characteristic with asimilar thermoplastic resin of the same fiow characteristics. It hasbeen found in the practice of sandwiching copper sheets betweeninsulating sheets of thermoplastic material initially having the sameflow characteristics, that when the base plastic sheet is laminated tothe cupric oxide coated copper sheet and then portions of the copper areremoved, that this base sheet requires a higher temperature to cause itto flow than before such treatment, and that by placing another sheet ofthe same plastic material not subjected to this treatment as a covercoat, the cover coat will then how first ut the original temperature. Inthis manner the base sheet does not flow at the initial laminatingtemperature and retains the copper configuration thereon such thatportions of the configuration do not move around relative to otherportions when the top cover coat is applied. This method of laminatingpermits very close spacing of conductive strips without making contactand is especially useful in the flexible printed circuit art. Toencapsulate copper between two sheets of thermoplastic material suchthat the material is of the same thickness above and below the copper ittherefore requires that the top sheet be slightly thicker than the basesheet, since the top coat must flow around the copper on the base sheetto intrude into the follicles of the base sheet. This flowing around andbetween the copper strips further serves to maintain the initial spacingtherebetween.

It has been found also in the application of the top sheet ofthermoplastic material that if the base sheet and copper bonded theretois first subjected to heat and pressure the copper will penetrate thefollicled surface. This again provides for a plastic material tomaintain the spacing between the copper strips. The copper may beembedded completely flush or below the surface as disclosed in myco-pcnding application, Serial No. 598,170, filed July 16, 1956, or itmay be depressed only partially and the top coat during its laminationwill flow down around and between the exposed copper. In this case thedetermination as to the thickness of the cover coat will depend upon thepenetration of the copper strips into the base material.

In the accompanying drawing:

FIGURE 1 shows in cross section the base material with copper oxidecoated copper strips bonded to the follicled surface and with the topthermoplastic material in position prior to its lamination.

FIGURE 2 is a sectional view of the completed article showing how thecover coat flows around and between the copper strips and into thefollicles on the top surface of the base material, and

FIGURE 3 shows a modification wherein the copper strips are partiallyembedded in the base material.

Referring now to FIGURE 1, a sheet of copper is coated with cupric oxide11 and 12 and then bonded to a base 13 under suitable heat and pressure.The cupric oxide has the characteristic that it will not flake off butwill effect a good bond. A desired configuration is then masked out onthe top surface of the copper and the exposed portion removed such as bychemical etching for example. The exposed surfaces from which the copperhas been removed from the plastic material will have follicles which hadbeen formed by an imprint of a cupric oxide when the copper was bondedto the plastic. A sheet of thermoplastic material 15 having the sameflow characteristics is shown placed over the conductors for lamination.Suitable examples of temperatures, pressures and procedures aredisclosed in my co-pending applications.

In FIGURE 2 there is shown in cross section the results of thelamination under suitable heat and pressure wherein the top sheetreached its flow point and flowed down between and around the copperconductors to become imbedded in the follicles of the lower sheet. Inthe finished product the irregular surface is still visible to the nakedeye when light is reflected at an angle, showing that the bottom sheethad not reached its flow point at the same temperature as the top sheet.If a still higher laminating temperature is desired, portions 16, 17 and18 of the top sheet 15 help to retain strips 10 in position. Theparticular reason or cause for this phenomena is not known although itis believed that perhaps the previously treated sheet has in some mannerchanged its flow characteristics to a higher temperature or that thefollicles present a greater exposed surface for cooling, thus requiringa greater temperature to make a lower sheet flow. As shown in FIGURE 2the thickness of sheet above the copper conductor will be less than thethickness of the material 13 below the conductors it initially the twothicknesses were the same. This must be compensated for by using a sheet15 of somewhat greater thickness if it is desired to center theconductors midway within the encapsulation.

In some instances there is a relationship between the plastic thicknessand the copper thickness such that any movement of the plastic will tendto force the conductors away from their original position during thecover coating operation. This excursion is controlled by impressing theuncover coated circuitry into the base plastic sheet as was mentioned inmy copending application, Serial No. 598,170. By imbedding theconductors downwardly into the base [platsic] plastic there is atendency to lock them into position and their movement is thuscontrolled. The resistance of this plastic along the sides of theconductors such that they will not move from the original desiredposition is shown in FIGURE 3 where the conductors 10 are imbeddedapproximately midway into the base sheet 13, leaving portions 19, and 21of the plastic material along the sides to oppose any lateral movement.In some cases this may be accomplished by pressure alone and in othercases heat and pressure is required so as to materially soften theplastic material and then permit it to solidify before applying the topcoat 15. In FIGURE 3 where the conductors are only partially imbedded,the top coat will flow around and between the conductors as shown inFIGURE 2 when laminated. However, if the conductors are imbedded flushwith the top surface of the base material the lamination [of] or fusionat the interface may be accomplished by using only sufiicient pressureand temperature to make the lamination less than that required to causethe cover coat to flow down between the conductors.

While there have been hereinbefore described What are at presentpreferred embodiments of the invention,

,it will be apparent that many and various changes and modifications maybe made to the embodiments illustrated without departing from the spiritof the invention. It will be understood, therefore, that all suchchanges and modifications as fall fairly within the scope of the presentinvention as defined in the appended claim are to be considered as apart of the present invention.

What is claimed is:

The method of bonding thermoplastic resins comprising the steps ofoxidizing a surface of a body of copper to provide a coating primarilyof black cupric oxide, placing said cupric oxide coated copper surfacein contact with the surface of a first thermoplastic resin having a flowcharacteristic at a given temperature, applying a sufficient degree ofheat and pressure to laminate said plastic resin and the coppertogether, forming thereby an imprint of said cupric oxide in saidplastic resin, removing portions of said cupric oxide coated copper fromsaid plastic resin to define a copper configuration and a plurality ofexposed follicles provided by said imprint in the exposed plastic,forcefully depressing said copper configuration into the follicledsurface until said copper configuration is partially driven into saidsurface of said first [coat] thermoplastic resin to thereby lock saidconfiguration in place to prevent conductor displacement during a.subsequent laminating step, placing a surface of a second sheet ofthermoplastic resin having a tendency to fiow at said given temperaturein contact therewith and over said partially driven copperconfiguration, and heating and pressing said plastic resins together toforce said second plastic resin into said follicles to provide said bondthere-between and form a laminate.

References Cited by the Examiner The following references, cited by theExaminer, are of record in the patented file of this patent or theoriginal patent.

UNITED STATES PATENTS 2,71 L983 6/1955 Hoyt. 2,932,599 4/1960 Dahlgren.2,964,436 12/1960 Mikulis et al.

EARL M. BERGERT, Primary Examiner.

J. H. STEINBERG, Assistant Examiner.

